PTFE (polytetrafluoroethylene) is a representative low-friction tribological material having the following characteristics.    (a) Melting Point: According to research publications, the melting point is constant at 327 degrees C. or has a range thereabouts.    (b) Meting Viscosity: Melting viscosity is as high as 1011 to 1012 Poise at 327 degrees C. Commercial PTFE dose not virtually flow even at the melting point or higher. Voids are therefore liable to remain after baking.    (c) Crystallinity: PTFE is classified as a crystalline resin and has high molecular weight.    (d) Processing Method: A porous sintered layer is impregnated with a PTFE dispersion consisting of finely divided PTFE particles, surfactant and water, followed by baking. The resultant product is used as a bearing.    (e) Structure: When the pre-baked PTFE is subjected to shear, it is easily fibrillated. The structure is apparently partially non-fibrillated after baking but the fibril structure is recognized when observed at high resolution. Therefore, the PTFE-based sliding material processed as (d) above has a fibril crystalline morphology.
Techniques for enhancing wear resistance of PTFE are largely classified into a method of adding an additive and a method for improving the structure of PTFE for itself. First, techniques for improving the wear resistance by additives are listed.
Patent Document 1: Japanese Unexamined Patent Publication (kokai) No. Hei 1-108413 proposes to impregnate a porous layer of backing metal with aromatic group polyetherketone, PTFE and metal fluoride and apply the aromatic group polyetherketone and the like on the porous layer.
Patent Document 2: Japanese Unexamined Patent Publication (kokai) No. Hei 8-41484 discloses a PTFE-based sliding material with the following additive components A, B and C: component A—phosphate, barium sulfate; component B—magnesium silicate or mica; and, component C—Pb, Sn, Pb—Sn alloy. In one example, a porous layer of a backing steel sheet is impregnated with PTFE and petroleum-based solvent, and the solvent is vaporized at 200 degrees C. Subsequently, pressure application by rolling is carried out. Heating is carried out at 370 degrees C. for 10 minutes, and pressure application by rolling is again carried out.
Pb exhibits excellent solid lubricant properties but its use is limited due to environmental problems. Therefore, Patent Document 3: Japanese Unexamined Patent Publication (kokai) No. 2002-20568 proposes addition of barium sulfate, molybdenum disulfide, graphite or the like to PTFE. In one example provided in this patent document, additives such as barium sulfate and PTFE resin (spherical colloidal hydrophobic resin obtained by emulsion polymerization) are mixed. A porous layer of backing metal is impregnated with the resultant mixture by means of a roll. Baking is carried out at 370 to 420 degrees C.
Next, prior art for improving the structure of a PTFE material are listed. Wear resistance and seizure resistance of the PTFE-based resin are unsatisfactory under such a severe conditions as in a main bearing of a compressor, which is subjected to cantilever load. Patent Document 4: Japanese Unexamined Patent Publication (kokai) No. 2007-09216 proposes a measure against the drawback described above, namely, twice baking to eliminate the boundary layer and to reinforce PTFE.
Patent Document 5; Japanese Unexamined Patent Publication (kokai) No. 2008-69196 belongs to none of the two classifications mentioned hereinabove. The method described in Patent Document 5 features that: PTFE contains a small amount of perfluoro octanic acid as an auxiliary agent of production; PTFE is dispersed by mean of an acrylic-resin-based dispersing agent; a porous layer of backing metal is impregnated with the resultant PTFE dispersion; and baking is carried out at 380 to 430 degrees C. The carboxyl group contained in the acrylic resin-based dispersion agent exerts a detrimental influence on the bearing performance. The time required for completely decomposing the acrylic resin at the baking temperature of PTFE is a few hours to a few tens of hours. As a result, complete decomposition of acrylic resin may not be realized. Therefore, the remaining amount of acrylic resin in the final product is specifically limited to 0.5% or less relative to PTFE.
A porous copper-alloy sintered layer is impregnated with a mixture of fluorine resin and solid lubricant, such as Pb, Zn, barium sulfate, graphite and the like. The mixture is dried and baked to provide a PTFE-based sliding material. This sliding material is frequently used in the production of a main bearing and a thrust bearing of a compressor for automobile air conditioner.